1. Field of the Invention
The present invention relates generally to ferromagnetic memory and more specifically to ferromagnetic memory utilizing giant magnetoresistance and spin polarization in annular memory elements.
2. Description of the Related Art
For many years, random access memory for computers was constructed from magnetic elements. This memory had the advantage of very high reliability, nonvolatility in the event of power loss and infinite lifetime under use. Since this memory was hand assembled from three-dimensional ferrite elements, it was eventually supplanted by planar arrays of semiconductor elements. Planar arrays of semiconductors can be fabricated by lithography at a much lower cost than the cost of fabricating prior art magnetic ferrite memory elements. Additionally, these semiconductor arrays are more compact and faster than prior art ferrite magnetic memory elements. Future benefits of increasingly smaller scale in semiconductor memory are now jeopardized by the concern of loss of reliability, since very small scale semiconductor elements are not electrically robust.
Non-volatile magnetic memory elements that are read by measuring resistance have been previously demonstrated by Honeywell Corporation. These systems operate on the basis of the classical anisotropic magneto-resistance phenomena, which results in resistance differences when the magnetization is oriented perpendicular versus parallel to the current. Previous work by others has shown that a 2% change in resistance is sufficient to permit the fabrication of memory arrays compatible with existing CMOS computer electronics. Unfortunately, scaling of these elements down from the current 1 .mu.m size has proved challenging.
The carriers in devices can be identified not only as electrons and holes, but also by their spin state being "up" or "down". Just as polarized light may be easily controlled by passing it through crossed polarizers, spin polarized electron current can be created, controlled and measured by passing it between magnetic films whose relative magnetic moments can be rotated. The spin polarization manifests itself as an extra resistance in a magnetic circuit element, commonly referred to as magneto-resistance. The modern manifestation of magneto-resistance should not be confused with older observations common to semiconductors and metal in which the carriers are merely deflected by the classical Lorentz force (V.times.B) in the presence of a magnetic field. This modern effect is purely quantum mechanical and occurs when two ferromagnetic metals are separated by a non-magnetic conductor. When a bias voltage causes carriers to flow from one magnetic metal into the other through the intervening conductor, the spin-polarization of the carriers can play a dominant role. The carriers leaving the first ferromagnetic metal are highly polarized because they are emitted from band states which are highly polarized. The resistance which they meet in trying to enter the second ferromagnetic layer depends strongly upon the spin polarization of the states available to them. If the ferromagnetic moments of the two magnetic metals are aligned, then the spin descriptions of the states are the same in the two materials and carriers will pass freely between them. If the two moments are anti-aligned, then the states are oppositely labeled (i.e., "up" in the first ferromagnetic layer is "down" in the second ferromagnetic layer), the carriers will find that they have fewer states to enter and will experience a higher resistance. This phenomenon is now commonly referred to as the spin-valve effect. By simply measuring the resistance between two magnetic layers, one can determine if their magnetic moments are parallel or anti-parallel.
Successful application of the spin-valve effect to magnetic memory elements would preferably minimize the cancellation of the magnetization in one ferromagnetic layer by fringing fields associated with the magnetization in another ferromagnetic layer.